sign published packages (#2674)
sign published packages (#2674)

Add signing plugin to sign artifacts using PGP. For details see
https://docs.gradle.org/current/userguide/signing_plugin.html

This was testing by publishing to local maven repository:
```
$ gradlew bookkeeper-server:publishMavenPublicationToMavenLocal

$ ls -ln ~/.m2/repository/org/apache/bookkeeper/bookkeeper-server/4.14.0-SNAPSHOT/
total 10568
-rw-r--r--  1 501  20  1596624 Mar 31 13:07 bookkeeper-server-4.14.0-SNAPSHOT-tests.jar
-rw-r--r--  1 501  20      650 Mar 31 13:07 bookkeeper-server-4.14.0-SNAPSHOT-tests.jar.asc
-rw-r--r--  1 501  20  2254279 Mar 31 13:07 bookkeeper-server-4.14.0-SNAPSHOT.jar
-rw-r--r--  1 501  20      650 Mar 31 13:07 bookkeeper-server-4.14.0-SNAPSHOT.jar.asc
-rw-r--r--  1 501  20     6983 Mar 31 13:07 bookkeeper-server-4.14.0-SNAPSHOT.module
-rw-r--r--  1 501  20      650 Mar 31 13:07 bookkeeper-server-4.14.0-SNAPSHOT.module.asc
-rw-r--r--  1 501  20     6251 Mar 31 13:07 bookkeeper-server-4.14.0-SNAPSHOT.pom
-rw-r--r--  1 501  20      650 Mar 31 13:07 bookkeeper-server-4.14.0-SNAPSHOT.pom.asc
-rw-r--r--  1 501  20     1817 Mar 31 13:07 maven-metadata-local.xml

```

signing can be disable using following using `ORG_GRADLE_PROJECT_skipSigning`:

```
ORG_GRADLE_PROJECT_skipSigning=1 gradlew :bookkeeper-server:publishMavenPublicationToMavenLoca
```

Co-authored-by: Surinder Singh <4f960c291c385df9684bdfb6d8650414a41bdc73@splunk.com>
apache/bookkeeper,apache/bookkeeper,apache/bookkeeper,apache/bookkeeper,apache/bookkeeper,apache/bookkeeper